1. Field of the Invention
The present invention relates to a method for purifying nitrogen oxide in an exhaust gas and an exhaust system operating the same. More particularly, the present invention relates to a method for purifying nitrogen oxide in an exhaust gas and an exhaust system operating the same which improve efficiency of purifying the nitrogen oxide as a consequence that the nitrogen oxide contained in the exhaust gas is stored and the stored nitrogen oxide is then released by fuel additionally injected when a predetermined condition is satisfied such that oxidation-reduction reaction between the stored nitrogen oxide and the additionally injected fuel occurs.
2. Description of Related Art
Generally, exhaust gas flowing out through an exhaust manifold from an engine is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matters (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DE-NOx catalyst) is one type of such a catalytic converter and purifies nitrogen oxide contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) are supplied to the exhaust gas, the nitrogen oxide contained in the exhaust gas is reduced in the DE-NOx catalyst through oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst is used as such a DE-NOx catalyst. The LNT catalyst absorbs the nitrogen oxide contained in the exhaust gas when the engine operates in a lean atmosphere, and releases the absorbed nitrogen oxide when the engine operates in a rich atmosphere.
An exhaust system using such an LNT catalyst controls operating state of the engine (lean or rich) according to nitrogen oxide amount stored in the LNT catalyst. However, in a case that the nitrogen oxide amount is detected and state of air-fuel mixture is controlled, regeneration timing of the LNT catalyst may be delayed. That is, though the control portion judges the regeneration timing, a substantially long time is required for meeting a target air/fuel ratio. Therefore, efficiency of purifying the nitrogen oxide may be deteriorated.
In addition, since regeneration timing of the DE-NOx catalyst is judged based on a signal transmitted from a NOx sensor, there may be a problem of malfunction of the sensor. In addition, expensive sensors may be used.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.